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Image Quality Improvement at the 0.9-m (1Dec95) (from CTIO, NOAO Newsletter No. 44, December 1995) We now have the data in hand to demonstrate the gain in image quality resulting from recent improvements to our dedicated CCD imaging telescope. The installation of fans at the primary mirror cell (April 1995) and new radial supports for the primary (June 1995) have produced several beneficial results. Some of the image astigmatism has been eliminated, and the focus is now nearly constant over the sky. Tests on 5-6 October showed focus changes of approximately +50 um as the telescope is moved from zenith to 50d-60d from zenith. This is a factor of two to four times smaller than previous focus changes. These improvements are being supported by funding from the MACHO project. Figure 1 shows a histogram of FWHM image diameters measured on focus frames over the past four months. During 59 nights, the median seeing is 1.25", the mean is 1.32", and the first quartile is at 1.04". Eight nights (13%) have FWHM less than 1.0", with the best images at approximately 0.8" arcsec. Previous seeing statistics tabulated by Nick Suntzeff from focus frames over a three-year period (335 nights, 1988-1991) gave a median seeing of 1.78" FWHM for the 0.9-m. My previous estimate of the number of nights with seeing < 1.0" on this telescope was about 1 night/year (0.3%), and the statistics from Suntzeff showed no nights with seeing < 1.0". The telescope is still far from perfect. The focus is very temperature dependent, about 35-40 um per degree C change. The temperature should be monitored carefully, particularly at the start of the night when the temperature change is large and focus frames are usually done. The leaky guider adds about 0.2" to the FWHM when used; a guider upgrade is under development. A coma corrector would help image quality over the full Tek2048 field. Current PSF variations require careful applications of quadratic terms; ~100 stars are needed to determine the PSF accurately. [Figure not included] Bob Schommer
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